1. Field of the Invention
The present invention relates to electrical wire and coaxial signal cable conduits with protective shielding, and more particularly to flexible lightweight conduits having conductive shielding layers to protect cables and wires from environmental conditions and to prevent electromagnetic interference (EMI) and radio frequency interference (RFI) from passing into and out from the conduit.
2. Description of the Prior Art
It is well known in the art that electrical signal transmission lines must be protected from other nearby lines and more generally from ambient electromagnetic interference (EMI) and radio frequency interference (RFI). EMI and RFI protection is of paramount importance in aircraft, and more so as they increasingly utilize new communication, navigation, radar, and computerized components critical to flight safety and economy. Such aircraft components and their associated cables and wires both emit and are vulnerable to EMI and RFI. Thus, aircraft manufacturers have a distinct interest in making aircraft with electronic components that operate free of EMI and RFI.
Aircraft manufacturers, in particular, are constantly seeking ways to reduce overall aircraft weight. This, too, increases flight safety and economy. Traditionally, the conduits used to house and protect electrical cables and wires for aeronautical applications and avionics have utilized heavy tin-plated copper metal overbraid for EMI and RFI shielding. The wire overbraid is comprised of strands of copper core wire plated with tin to protect against corrosion. However, an overbraid has several limitations. Most notably, the large number of strands needed to cover a conduit with the 90 percent coverage required for proper shielding is the largest single weight component of shielded conduits. Thus, it is desirable to have a lightweight means of providing effective EMI and RFI shielded conduits.
Another limitation of overbraid concerns its lack of longevity: as is well known in the field, the performance of overbraid deteriorates over time through flexing and vibration. Flexing in particular causes the copper in the braids to work harden and eventually to break. Thus, an effective but more durable EMI/RFI shielding for conduit is desirable.
Finally, metal overbraid allows for very limited control of the degree of shielding effectiveness. The amount of shielding can be increased only in large increments by adding complete layers of overbraid. This limits the ability to minimize weight by tailoring the conduit""s EMI shielding effectiveness to its specific requirements. Thus, a more adaptable shielding technique is desirable.
One solution to the weight problem consists of jacketing conduit with wraparound metal foils laminated onto thin plastic tape. Shielded cables are fabricated by winding the foils around wires or a conduit. However, metal foils are generally not rugged enough to withstand the flexing and vibration of many conduit applications. So even though the foil does provide lightweight shielding, its effectiveness also diminishes as it is flexed and handled.
Another technique for providing EMI/RFI shielding involves using light cloth fabrics plated with a thin coating of metal. Metalized fabrics provide excellent EMI shielding properties, largely because the metal may be applied only where needed on the surface of the material. In addition, the cloth can be tightly woven to prevent the gaps and holes that often result in EMI penetration. For these reasons, EMI shielding effectiveness per pound of material of cloth is superior to the metal overbraid traditionally used in cables and conduits, and metalized fabric has been used extensively and successfully to provide EMI shielding on rigid electronic equipment housing.
However, metalized fabric also suffers certain limitations. While eminently suitable for rigid housing, metalized fabric, as currently used, is considerably less effective with flexible cables and conduits. This is due to the fact that the plated metal tends to flake or peel from the fabric substrate when the cloth is creased or folded. Even though the cloth can be tightly wrapped around rigid conduits, when applied to flexible conduits, the tightly woven cloth is forced to crease and fold to accommodate bends in the conduits. The result is a conduit that loses effectiveness over time.
Hoses of the inventive type are known in the art. For instance, U.S. Pat. No. 5,393,928 to Cribb, et al., entitled, Shielded Cable Assemblies, teaches lower weight shielded cable assemblies with an enhanced level of shielding effectiveness, comprising a core of at least one insulated conductor element overwrapped with metallized fabric, e.g. characterized as having a surface resistivity less than 100 milliohms/square or as being a metallized fabric coated with at least a layer of copper having a metal density of greater than 50 grams/square meter. The inventive cable assemblies may employ a shielding subassembly comprising braided wire and one or more layers of copper-metallized fabric where the shielding subassembly has a transfer impedance at 10 MHz of less than 50 mo/m. Cribb et al note that it has been found that cable assemblies employing a four layer wrap of certain metallized fabrics can provide up to 20 decibels improvement in shielding effectiveness over a wide range of frequencies with a 74 percent reduction in weight compared to a standard wire braid/foil laminate shield.
However, it should be noted that Cribb discloses nothing more than shielded cable assemblies having an insulated core element overwrapped with metalized fabric. There is not a single drawing in Cribb et al disclosing a preferred embodiment of the shielded assemblies. Rather, Cribb commends the use of metalized cloth as a shielding method. The problem in this simple solution is that flexing of the cable causes the metal plating to flake off and ultimately renders the device useless for shielding. This is a primary problem solved by the shielded conduit of the present invention.
U.S. Pat. No. 4,342,612, to Lalikos et al discloses semirigid, convoluted plastic hoses made from material such as polytetrafluoroethylene with a spiral wire spring wrapped thereon, primarily, although not exclusively, for use as a vacuum hose. The spring wire is pulled off the end of a supply bobbin and looped around a pulley or idler spool, which is a coil forming wheel having a diameter that is correlated to the diameter of the plastic hose. The wire is then pulled from the end of the forming wheel, over a controlled radius guide or shoe, to the hose surface. The looping on the forming wheel, followed by the pulling therefrom via the circular guide, gives the wire a permanently preformed circularized and spiral shape. The resulting spiral shape enables the wire to snap into the hose convolutions as it is wound directly onto the plastic hose. The relative diameter of the wire, the width of the space between the convolutions, and the diameters of the forming wheel and plastic hose are such that the wire is securely captured within the valley of the convolutions after it snaps into position. This provides the hose with dramatically increased strength to withstand substantial vacuum pressures without danger of collapsing and to withstand crushing forces from over-bending and being stepped on.
The hose disclosed in Lalikos et al is adapted for use as a vacuum hose, i.e, non-electrical hose. It is ill-suited for electrically terminating an EMI shielded conduit as it does not provide any means of EMI shielding.
U.S. Pat. No. 4,112,246, to Dembiak et al., teaches a gas feeder pipe assembly or conduit comprising a central cylindrical aluminum sheath which gives shape to a duct for carrying fluid under pressure. The sheath is coated on both sides with an adhesive film, such as a polyethylene laminate. The film bonds an overlap of the sheath after it is rolled into its cylindrical shape, and bonds an extruded polyethylene jacket to the duct formed of the sheath. The aluminum sheath in combination with the jacket results in a pressurable pipe for carrying fluids, such as dry air, in a manner consistent with prior art practices. A plurality of insulated conductors are applied around the duct in a peripheral layer. The conductors are further applied around the periphery of the duct in an oscillating lay, the lay establishes an alternately clockwise and counterclockwise oriented skew in the conductors with respect to a central axis of the duct. The lay gives additional length to the conductors with respect to the length of the duct to avoid stresses in the conductors due to variations in expansions or contractions between the duct and the conductors. The conductors are not embedded into, or otherwise made part of, the polyethylene jacket of the duct. Instead, the jacket serves as a smooth supporting surface for the peripheral lay of the conductors. The conductors are slideably held in place on the surface by a binder. An insulating polyethylene corewrap film is applied over the lay of the conductors. The film has a longitudinal overlapping portion.
Although Dembiak et al disclose a conduit having conductive bare wires and a method of incorporating the wires into the lamination around a metalized fabric tape with a slight helical wrap to act as a ground for the conduit, the wires are laid directly on the smooth surface of a conduit. This does not solve, but merely exacerbates the problem in flaking of the metalized fabric cloth. Furthermore, in numerous applications, metalized cloth is insufficient to ground cables carrying high voltage.
The lightweight shielded conduit of the present invention solves the problem of flaking and addresses the problem of using metalized fabric alone as a shield. By using metal wire to pull metalized cloth into the troughs of a conduit having standard helical convolutions, the wire becomes a helical ground wire which conducts the current to ground, rather than to the metalized cloth. This configuration entails the use of metalized cloth in different configurations to ensure long life in the shielding material and thus effective shielding over time for a variety of voltages and frequencies of shielding required under the specific application.
The lightweight shielded conduit of the present invention includes an inner core of elastically deformable flexible tubing having an outside surface of helical corrugations. Metalized fabric is wrapped around the helical corrugations of the inner core in a substantially helical fashion, and is then post-formed onto the corrugated helix of the core. The forming process preferably involves wrapping a string or wire around the fabric so as to pull the fabric down into the base of the continuous trough of the corrugations. An axial drain wire may be incorporated into the lamination when lower ground path resistance is required. This wire runs length of the conduit with only a slight helical wrapping to maintain conduit flexibility. The corrugated inner core, metalized fabric formed onto the inner core, and drain wire (if included) are encased in a protective jacket.
In a second preferred embodiment of the present invention the axial drain wire is eliminated and a second layers of shielding material is included, separated from a first layer by a layer of polyester foil. An electrically conductive reinforcement wire is wrapped around the conduit, compressing the cloth and foil tape into the convolute depressions of the helical convolutions, and a layer of shrink tubing is applied the cover the assembly.
Another embodiment of the present invention is identical to that of the second embodiment, excluding the intermediate foil layer.
The present invention has several advantages over metal overbraid and the current use of metalized fabrics. It is extremely lightweight, has increased flexibility and tolerance to vibration, has increased EMI/RFI shielding. Other advantages of using the present invention include the following: (1) metalized cloth may be formed into a flexible conduit so as to prevent flaking and delamination of the metal from the fabric substrate; (2) shielded conduit can be produced in long continuous lengths using the invention; (3) a conduit with a thinner wall than braided conduit can be produced, resulting in a lower conduit profile; (4) the degree of shielding can be precisely controlled at the time construction; and (5) shielding effectiveness can be continuously tailored and optimized to reduce weight at the time of manufacture.